The subject matter disclosed herein relates generally to gear pumps, and more particularly, to a bearing and gear arrangement for a gear pump used in an aircraft Fuel Metering Unit (FMU).
Gear pumps often include two gears, usually of a similar configuration, that mesh with each other inside of a housing. A first or drive gear is an extension of a drive shaft; as it rotates, it drives a second gear, commonly referred to as the driven gear. As the gears rotate within the housing, fluid is transferred from an inlet to an outlet of the pump. Either spur gears or helical gears may be used, however, spur gears are most common.
The volume of fluid pumped through a gear pump depends partially on the depth of the tooth, the tooth diameter, and the width of the gear. Larger volumetric output is realized when fewer gear teeth having large working tooth depths and face widths are used. As the spur gear rotates, individual segments of fluid are released between the teeth to the outlet, thereby pulsing or rippling the output flow.
When larger gear pumps are operated at extremely high speeds, the surfaces of the gear teeth and bearings experience cavitation erosion. Cavitation erosion results in the pitting of the surfaces of the gear teeth and bearings and can eventually result in degraded pump volumetric capacity and durability. As the gears rotate through their mesh cycle, an opening inter-tooth volume is formed. As the meshing zone volume increases in size from minimum to maximum, the pressure within this meshing zone volume decreases below the true vapor pressure and vapor bubbles or cavities are formed. As the gears continue to rotate and the volume is further filled, this volume is subjected to higher pressure. As a result, the vapor formed within the inter-tooth volume is either forced back into the solution or collapsed as the cavitation phenomena. The vapor collapsing energy associated with this phase change is significant, and the vapor cavity, in effect, implodes on the gear teeth and bearing surfaces, thus resulting in cavitation erosion damage.